In the nervous system, neurons propagate signals via action potentials. These are brief electric currents which flow down the length of a neuron causing chemical transmitters to be released at a synapse. The time-varying electrical current within the neuron generates a magnetic field, which propagates through the human body and can be measured using either a Superconductive Quantum Interference Device (SQUID) or an Optically Pumped Magnetometer (OPM). In this disclosure the OPM is primarily considered because the SQUID requires cryogenic cooling, which may make it prohibitively costly for users and too large to be wearable by a user. In addition to OPMs and SQUIDs, other magnetic sensing technologies for detection of magnetic fields from the brain include and magnetoresistance.
Optical magnetometry can include the use of optical methods to measure a magnetic field with very high accuracy—on the order of 1×10−15 Tesla. Of particular interest for their high-sensitivity, an optically pumped magnetometer (OPM) can be used in optical magnetometry to measure weak magnetic fields. In at least some embodiments, the OPM has an alkali vapor gas cell that contains alkali metal atoms in a combination of gas, liquid, or solid states (depending on temperature). The gas cell may contain a quenching gas, buffer gas, or specialized antirelaxation coatings or any combination thereof. The size of the gas cells can vary from a fraction of a millimeter up to several centimeters.